Cochlear implantation is a surgical procedure performed on individuals who experience profound to severe sensorineural hearing loss. In cochlear implant (CI) surgery, an electrode array is permanently implanted into the cochlea by threading the array into the basal turn, as shown in FIG. 2. The array is connected to a receiver mounted securely under the skin behind the patient's ear. When activated, the external processor senses sound, decomposes it (usually involving Fourier analysis), and digitally reconstructs it before sending the signal through the skin to the internal receiver, which then activates the appropriate intracochlear electrodes causing stimulation of the auditory nerve and the perception of hearing. Current methods of performing the surgery require wide excavation of the mastoid region of the temporal bone. This excavation procedure is necessary to safely avoid damaging sensitive structures but requires surgical time of at least 2 hours. Recently another approach has been proposed—percutaneous cochlear access—in which a single hole is drilled on a straight path from the skull surface to the cochlea [1,2]. The advantages of this technique are time saving and uniform insertion of the electrode array, which may be less damaging to inner ear anatomy.
Percutaneous cochlear access is accomplished by using preoperative CT images to plan an appropriate drilling trajectory from which a drill guide is rapid-prototyped. Prior to surgery, in the clinic, anchors are affixed to the patient's skull by creating three small skin incisions and screwing self-tapping anchors into the bone. The incisions are stitched over the anchors, and a CT image of the patient is acquired. Using the image, a safe direct drilling trajectory from the skin surface to the cochlea is then selected. The anchors are localized, and a unique platform, which is used as a drill guide, for example, STarFix™ microTargeting™ Platform (FHC, Inc., Bowdoin, Me.), is manufactured for each patient using software designed to mathematically relate the location of the bone markers to the trajectory. On the day of surgery, the platform is mounted on the anchors and the drill is mounted on the guide. FIG. 3 shows a picture of the platform mounted on a patient's skull with a drill bit in place.
One major difficulty with the percutaneous approach is the selection of a safe drilling trajectory. The preferred trajectory passes through the facial recess, a region approximately 1.0-3.5 mm in width bounded posteriorly by the facial nerve and anteriorly by the chorda tympani, as shown in FIG. 2. The facial nerve, a tubular structure approximately 1.0-1.5 mm in diameter (about 3 voxels), is a highly sensitive structure that controls all movement of the ipsilateral face. If damaged, the patient may experience temporary or permanent facial paralysis. The chorda is a tubular structure approximately 0.3-0.5 mm in diameter (about 1 voxel). If the chorda is damaged, the patient may experience loss in the ability to taste. In the previous work studying the safety of CI drilling trajectories, in which error of the drill guide system was taken into account [4], it is determined that safe trajectories planned for 1 mm diameter drill bits generally need to lie at least 1 mm away from both the facial nerve and the chorda. Fitting a trajectory in the available space can thus be difficult, and any planning error can have serious consequences.
During the planning process, the physician selects the drilling trajectory in the patient CT by examining the position of the trajectory with respect to the sensitive structures in 2D CT slices. This is difficult, even for experienced surgeons, because the size of the facial nerve and chorda and their curved shape makes them difficult to follow from slice to slice. The planning process would be greatly facilitated if these structures could be visualized in 3D, which requires segmentation. Segmentation of these structures is also necessary to implement techniques for automatically planning safe drilling trajectories for CI surgery [4].
Atlas-based segmentation is a common technique, which relies on image registration, to perform automatic segmentation of general structures in medical images. But one underlying assumption on which these methods are based is that the volumes to be registered are topologically equivalent. Even when this is the case, these methods are challenged by applications in which lack of local contrast makes intensity similarity measures used to drive the algorithms ineffectual. In the percutaneous cochlear implementation, both the lack of local contrast and topological differences are issues. Indeed, the facial nerve and chorda are surrounded by structures of comparable intensity values and pneumatized bone. Pneumatized bone appears as voids within the bone, which can be vary in number and location across subjects. Because of these characteristics, atlas-based methods alone do not lead to results that are accurate enough.
FIGS. 4 and 5 show that purely intensity based segmentation methods are unlikely to be successful. Difficulties include partial volume effects due to the size of the structure, lack of clearly defined edges, and changes in the intensity characteristics along the structures' length (dark in some section and bright in other sections). One notes that the structures of interest are tubular, and a large body of literature has been published to segment this type of structure [5-11]. Among these methods, a minimal cost path based approach [9-11] extracts the structure centerline as the path of minimum cost from a starting to an ending point through the image. Typically, the cost function used to associate a cost to every connection between neighboring voxels involves terms derived from the image, such as intensity value or gradient. In general, however, the cost function is spatially invariant, i.e., it is computed the same way over the entire image. Spatial invariance is a severe limitation for the percutaneous cochlear implementation because the intensity characteristics of the structures of interest vary along the structures. Additionally, these methods also do not use a-priori geometric information, which is also a limitation for the percutaneous cochlear implementation because the path of minimum cost, based on intensity values alone, does not always correspond to the structure centerline.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.